There's an extended scene in 300 in which Gerard Butler -- massive and fearless in the role of legendary Spartan king Leonidas -- slices and dices his way through the invading Persian hordes, laying low some 20 of the enemy in the process.

Refusing to let his stunt double take over here, Butler was celebrating his own sense of empowerment, helping re- enact a key moment in one of military history's greatest confrontations -- the ancient Battle of Thermopilae which saw Leonidas and 300 Spartan warriors engage in a fight to the death to defend their homeland against Xerxes and his massive Persian army.

Thanks to the wonders of state-of-the art visual and computer effects and to the latest green-screen techniques, you wouldn't know that this sequence -- so evocative of the rugged, towering grandeur of the Aegean cliffs and so redolent of the dust and heat and fury of battle -- was actually shot within the sprawling confines of a former railway factory in Montreal. As for Butler, this was the moment where he became genuinely immersed in his character and in both the reality and the mythology of ancient history.

"I've never come across a character quite as powerful and intense and charismatic as this guy -- and as bad-ass," the Scottish-born actor says. The Warner Bros. film, which opens Friday, is inspired by Frank Miller's acclaimed graphic novel, and Butler knows that this factor moves Leonidas's heroic story into contemporary dimension.

Butler, whose last major project was the title role in the film version of Phantom of the Opera, spent seven months of intense physical training preparing for 300 and the role of Leonidas. He was also guided by the vision of costume designer Michael Wilkinson, who went directly to the Miller novel which visualized the Spartan soldiers as magnificent physical specimens with capes flung dramatically around their muscular torsos. In the case of Leonidas, Butler knew the king's physicality helped define his leadership. He found himself working out before every take and eventually had the sense of being this man.

"Every time I trained, it made me feel more like a Spartan, more like a king, more like I was impressing my men, and more like they would be willing to follow me. Also, that fire is burning inside you. . . . I literally walked around Montreal with my shoulders back and my chest up."

Butler knew he had to go beyond creating a one-dimensional mythic hero. "He has a lot of things going on. There's an arrogance there, there's a confidence, there's a humour, there's a dryness, there's a compassion, there's a certain amount of humanity."

However, adds Butler, there's even more to Leonidas. "The guy is a nut job. He's crazy and there's a fearlessness that borders on insane. To try and get all those in with a man who really doesn't talk that much was a challenge. And then, to do it all in front of green screen. . . ." Butler shakes his head. "As you can see, I'm really not good at talking about it. I just do it."

What keeps coming through in this morning's conversation is that in shaping the role, Butler went beyond total immersion -- whether engaging in hand-to-hand conflict with the enemy, or defending the principles of freedom and honour in a dramatic meeting with Brazilian actor Rodrigo Santoro's godlike Xerxes or indulging in a steamy sexual encounter with Lena Headey's sultry queen. He admits that the role became an addiction.

"After a certain point, I never felt silly or strange standing in my cape. That started to become one of my strongest allies. Wearing that costume and feeling so strong was exciting, knowing that your body was also an intimidating factor and an inspiring factor for your army. You're surrounded by probably a few tons of muscle and when you pool that and pull all that spirit together, and have nothing but focus and belief and pure intention, with the power of all that, you become 1,000 times stronger. So it actually makes sense that you could confront an invading army that doesn't have that belief."

The 37-year-old actor had an almost "psychic feeling" about the project from the beginning. He was hooked as soon as he heard of plans for a movie called 300. He didn't know then what it was about "but the title 300 was so simple and strong -- like a strong guy with a shaved head."

Butler sees it as a positive sign that the producers had trouble securing financing, "because you know that means you're not making something mainstream.

"You're making a vision and that vision really has to be impressed on people and people have to be turned on and clicked into what that vision really is."